Electrolytic polishing of metals



Patented Apr. 22, 1952 2,594,124 ELECTROLYTIC POLISHING OF METALS PercyAllan Charlesworth, Adlington, near Macclesfield, England No DrawingApplication September 2, 1947, Se-

rial No. 771,591. 12, 1946 1 Claim. (01. 204-1405) This inventionrelates to improvements in the electrolytic polishing of metal surfaceswherein an electrolyte of sulphuric acid or of a mixture of sulphuricacid and phosphoric acid, the electrolyte being of high acidconcentration, is employed and the. article to be polished is made theanode.

The generally accepted theory is that an anodic film is formed on thesurface of the metal being polished, through which film the high spotsproject and are dissolved away resulting in the formation of a smoothsurface.

The method of polishing metals and metal alloys byimmersing them in anelectrolyte and passing an electric current therethrough, utilising thearticle to be polished as the anode and employing as the cathode eitherthe tank containing the electrolyte or an independent or conformingcathode, has in general, as is wellknown, many advantages over themechanical methods of polishing. It utilises non-skilled labour, isquicker and cleaner than the mechanical method, articles once they havebeen introduced into the solution can be left unattended until it istime to withdraw them from the electrolyte, in most cases oneelectrolytic treatment is suflicient to obtain the required degree ofpolish and the process lends itself to the mass production of polishingof large outputs.

These advantages are particularly great when an electrolytic polishingprocess is applied to the polishing of stainless steels since thesemetal alloys are in most cases very difficult to polish mechanicallyandfor their successful polishing require the use of skilled labour andlong and arduous abrasive treatment with difierent sizes of grits andfinally mopping. Even when a bright surface is obtained thesurfacelayerhas been greatly worked,"piled, heated, to form an amorphous orpseudo amorphous layer. Moreover particles of abrasive are often driveninto the metal and are subsequently responsible for pit corrosion. 7 Q

The electrolytic method of polishing has been described in the technicalliterature and the specifications of a number of patents describevarious electrolytes for use in the method but so far as the applicantis aware none of these various electrolytes are suitable or economicalfor electropolishing large articles on a commercial scale.

Among prior specifications relating to electrolytes for the electrolyticpolishing of metals is the specification of EnglishfPatent l \lo.-526,854

which describes an electrolytic polishing method wherein the metal to bepolished is made the In Great Britain September anode and theelectrolyte contains sulphuric acid and phosphoric acid of a combinedconcentration of at least 50% by weight of the electrolyte and anelectric current is passed therethrough of suflicient density and for asufficient length of time to produce a polished surface of the metal.The composition of the electrolyte described and claimed in thespecifiction lies within the wide range of 15% to sulphuric acid and 15%to phosphoric acid. 7

Experiments carried out by the applicant show that the aforesaid rangecovers mixtures of sulphuric acid and phosphoric acid which do not givea satisfactory polish within reasonable limits of current density andtemperature, i. e. a current density below 2 amps. per square inch and atemperature not exceeding about 50 C. and in some cases etching occursover the whole surface immersed in the electrolyte. Speaking generallyunsatisfactory results are obtained when the ratio of sulphuric acid tophosphoric acid exceeds 5 3. r

The aforesaid specification No. 526,854 infers that the best results areobtained when the relative proportions of sulphuric acid and phosphoricacid are 15% to 20% sulphuric acid and 63% to 67% phosphoric acid but itis found that when the proportion of phosphoric acid is high theresistance of the solution rapidly increases with use and drivingpotentials in the region of 50 volts are necessary to obtain areasonable life from the electrolyte.

The use of an electrolyte based on sulphuric acid alone is not acommercial proposition since although there is a tendency to polish inan aqueous solution of 60% by weight of sulphuric acid with high currentdensities in the range of 3 amps. per sq. inch and upwards, much sprayis thrown off by the solution which is an inconvenience in practice. Athigher concentrations of acid less spray is evolved but the tendency ofsalts to crystallize out on the metal is great and spoils the polish.When high current densities are used for polishing on a large scale veryeflicient cooling is required.

Other baths have been suggested making use of anaqueous solution ofphosphoric acid and containing additions of glycerine (20% or more).

These solutions are expensive and unless the metal undergoing polish hasa fairly smooth finish to start with, treatment times require to beprolonged. Treatment times can be shortened by -increasing operatingtemperatures e. g. C.

' The solutions tend to absorb or over but this introduces practical difilculti es.

moisture very'freely from the air and thus become unbalancedparticularly when not in use. The solutions are also viscous therebyincreasing drag-out losses and introducing other objections inherent insuch solutions.

As the solutions are worked resistance increases and the working voltageto obtain reasonable life of the solution may have to be increased to asmuch as 50 volts. High voltages of course, introduce many difficulties.Temperatures due to working become excessive and external cooling has tobe very efiicient and thus expensive, the solutions eventually becomesaturated with salts and no longer capable of polishing andnecessitating expensive and elaborate generating apparatus andelectrical gear.

Other electrolytes based on sulphuric acid and carboxylic acids e. g.citric acid) have been suggested but such are relatively expensive andunless the solution is kept warm the organic acid tends to be thrown outof the solution.

The object of the present invention is to provide a method for theelectrolytic polishing of metals employing an electrolyte which will beless expensive than those which it has hitherto been found possible towork on a commercial scale, which can be regenerated, which will ingeneral give faster working at given current densities and which can beemployed within a temperature range of 45 C. to 60 C. A temperaturewithin this range will not burn the operative and has many otherpractical advantages.

The electrolyte is primarily intended for the electrolytic polishing ofstainless steels but they can be employed for polishing some othermetals and alloys by suitable adjustment of the operating condition.

The electrolyte according to the invention consists of an aqueoussolution of sulphuric acid and phosphoric acid with a relatively smallamount of an additive consisting of aniline.

The acid concentration should be between 50% and 80% by weight of thesolution and the ratio of sulphuric acid to phosphoric acid in the caseof a. mixed solution should be greater than :3. Suitable ranges ofsulphuric acid are 4.0% to 70% by weight and of phosphoric acid 0 to 30%by weight.

Aniline is effective at concentrations of from 0.3% to 3% by weight.

Suitable current densities when using an electrolyte according to theinvention are between 0.5 amp. and 1.5 amps. per square inch of thesurface of the metal forming theanode and the voltage in general liesbetween 3 volts and volts.. A satisfactory electrolyte workingtemperature is 50 C. to 60 C. which is a very convenient range from acommercial standpoint. The time of treatment varies according todifferent factors and the initial condition of the surface of the metaland may be between a-few minutes and half an hour.

The following are typical examples illustrating the invention. In theseexamples the sulphuric acid is commercial sulphuric acid (95% H2SO4specific gravity 1.84) and the phosphoric acid is commercialortho-phosphoric acid (85% H3PO4 specific gravity 1.7). It is of coursepossible to replace ortho-phosphoric acid by suitable proportions ofmeta-phosphoric acid or pyro-phosphoric acid. The percentage of theadditive is the pure chemical.

4 In all the examples the parts are by weight.

Examplel Sulphuric acid 55 Phosphoric acid; 22 Water 23 Aniline 0.5

' Example 2 Sulphuric acid 62 Phosphoric acid 15 Water 23 Aniline 0.5

Example 3 Sulphuric acid '70 Water 30 Aniline 1.5

Y Example 4 Sulphuric acid 60 Water 40 Aniline 1.5

With baths based solely on sulphuric acid or where the proportion ofthis acid to phosphoric acid is high, it is advisable to provide acontinuous filtration unit on the polishing tank and in some casescontinual agitation is an advantage.

Other advantages of the use of aniline in addition to its relativecheapness are:

(a) It has a long life before its effects Wear off.

(2)) When polishing large areas a uniform degree of polishing isobtained over the whole surface.

(c) When polishing stainless'steel on passing the current a blue anodicfilm is formed on the surface of the metal which can be removed bywashing. This blue colouration serves as an indicator of the extent towhich the adhering drag-out electrolyte has been washed off.

Although the invention has been mainly described in its application forthe electrolytic polishing of stainless steel, it can be used forpolishing other ferrous base metals and alloys, and nickel and itsalloys.

. The invention has the following advantages over other known processesfor polishing stainless steel and other metals.

' 1. Relative cheapness of the electrolyte since the proportion ofphosphoric acid can be kept low or in some cases phosphoric acid can beomitted.

2. When salts are formed due to dissolution of the metal undergoingtreatment depending to some extent on the composition of theelectrolyte, they begin to precipitate out after a time and can befiltered off.

3. Solutions will polish at relatively low current densities of 0.3 amp.per square inch and upwards. It is found that current densities inexcess of 1 amp. per square inch are seldom necessary in solutionscontaining phosphoric acid. In solutions based on sulphuric acid alonehigher current densities may be advisable but it is found that littleimprovement in polish is obtained with a current density over 2-3 amps.per square inch.

4. Owing to the high conductivity of aqueous solutions containingrelatively large amounts of sulphuric acid, low driving potentials canbe used e. g. 3 'to 10 volts are adequate forpractically all purposes.Moreover large increases of required driving potentials are not found tobe necessary after the solution has been in use for long periods.

' 5. The electrolytic solution can be regenerated by filtration andfortification with fresh acid or .wate'r; f' l 6. The, solutions are ofrelatively low viscosity and the use of the additive does not rely forits bcneficial' feifect on increasing the viscosity of the body of theelectrolyte although it induces desirablegcliaracteristics in the anodiclayer.

7. The electrolyte can be used commercially in tanks holding up to 5tons and articles with areas as great as 20 square feet can besatisfactorily polished.

I claim:

A method for the electrolytic polishing of.

metals employing an electrolyte consisting essentially of an aqueoussolution of sulphuric acid and phosphoric acid, which comprisesimmersing the article to be polished as the anode in the acid solutionhaving an acid concentration of between 50% and 80% by weight of thesolution and a ratio of sulphuric acid to phosphoric acid greater 6 than5 to 3, and which contains an additive or between 0.3% and 5% by weightof the solution and consisting of aniline.

PERCY ALLAN CHARLESWOERTH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,334,698 Faust Nov. 23, 19432,348,517 Beckwith May 9, 1944 2,411,410 Beckwith Nov. 19, 1946 OTHERREFERENCES "Journal of The Electrodepositors Technical Society, vol. 21(194546), pages 256-258.

